Blade pitch measurement is desirable for efficient operation of propeller driven aircraft. An aircraft propeller has a number of pitched blades which rotate around a hub to produce thrust. The amount of thrust produced by the propeller is a function of the rotational speed of the propeller and the pitch of the blades. In order to maintain an efficient rotational speed while changing the amount of thrust, the pitch of the blades must be adjusted. However, the blade pitch must be known to properly adjust the pitch of the blades.
It is known in the prior art to use toothed wheels or their equivalents to measure blade pitch. Typically, two toothed wheels--a reference wheel and a variable wheel--are used. Each wheel has corresponding sets of equally spaced teeth which serve as reference points. The reference wheel turns on a shaft at a known rotational speed and the variable wheel turns on another shaft at substantially the same rotational speed as the reference wheel. The variable wheel is connected to the propeller in such a way that as the blade pitch changes, the variable wheel advances or retreats, i.e., deflects, slightly relative to the reference wheel. Fixed position magnetic pick-ups located proximately to both wheels sense the passing teeth on the wheels. The magnetic pick-ups generate waveshaped pulses when the teeth pass by. Changes in deflection are reflected as changes in the timing between waveshaped pulses. Thus, blade pitch can be calculated from the relative deflection of the teeth of two toothed wheels.
Due to manufacturing constraints, the spacing between teeth on a wheel varies slightly. It is not possible to produce a perfect wheel. It naturally follows that it is not possible to produce two perfect wheels with equally spaced teeth to serve as a variable wheel and reference wheel.
Tooth spacing imperfections present a problem for blade pitch measuring systems. On a per revolution basis, the measurement errors caused by such spacing imperfections will cancel because every wheel has 360 degrees. In other words, even though tooth spacing may vary from tooth to tooth, the sum of all inter-tooth spacing measurements must still add up to 360 degrees. However, measuring on a per revolution basis results in an unsteady transient output. As the blade pitch changes, the measured relative deflection jumps from one reading to the next due to the time lag imposed by waiting for a full revolution. Uncorrected measurements taken on a less than once per revolution basis, i.e., including the error, provide both an unsteady steady-state and an unsteady transient output.